Why should I step down in the speaker area to keep a small amp? I will be making less bass to match a sub to that amp. Id rather either get another amp so i can run two amps ad two subs or run a larger amp to power those 2 12s. Seems like this amp i have is more ideal for one speaker anyway.
Adding another sub and amp isn't always the way to get more output. It all depends on which subwoofer you get, the proper enclosure built, and an amp to match the specs. All those combined can give you the much needed output you seek
Obviously you want to go loud and not necessarily sq. You'll get very different opinions. Running one amp to it will get you better sq and faster response while smoothing out the acoustic "echo". One good sub paired with a great amp will always do better than two average. Invest in a crossover for that sub.according to the music you listen to and the music will improve ten fold.